Recently, the demand for portable electronic products such as notebooks, video cameras, cellular phones or the like has rapidly increased, and electric vehicles, energy storage batteries, robots, satellites have been actively developed. For this reason, high-performance secondary batteries allowing repeated charging and discharging are being actively studied.
Currently, nickel-cadmium batteries, nickel-metal hydride batteries, nickel-zinc batteries, lithium secondary batteries, and the like are used as commercial secondary batteries. Among them, lithium secondary batteries have little to no memory effect in comparison with nickel-based secondary batteries, and thus lithium secondary batteries are gaining a lot of attention for their advantages of free charging or discharging, low self-discharging, and high energy density.
A lithium secondary battery generally uses lithium oxide and carbonaceous material as a positive electrode active material and negative electrode active material, respectively. The lithium secondary battery includes an electrode assembly in which a positive electrode plate and a negative electrode plate respectively coated with the positive electrode active material and the negative electrode active material are disposed with a separator being interposed between them, and an exterior, namely a battery case, which seals and accommodates the electrode assembly together with an electrolyte.
Generally, a lithium secondary battery may be classified into a can-type secondary battery where the electrode assembly is included in a metal can and a pouch-type battery where the electrode assembly is included in a pouch of an aluminum laminate sheet, depending on the shape of the exterior.
Recently, secondary batteries are widely used not only for small-sized devices such as cellular phones but also middle-sized or large-sized devices such as vehicles and power storages. In particular, along with the exhaustion of carbon energy and the increased interest on environments, hybrid electric vehicles and electric vehicles attract attention globally, for example in US, Europe, Japan and Korea. In such a hybrid electric vehicle or electric vehicle, a battery pack for giving a driving force to a vehicle motor is the most essential part. Since a hybrid electric vehicle or electric vehicle may obtain a driving force by means of charging and discharging of the battery pack, the hybrid electric vehicle or electric vehicle ensures excellent fuel efficiency and exhausts no or reduced pollutants, and for this reason, hybrid electric vehicles and electric vehicles are used more and more. In addition, the battery pack of the hybrid electric vehicle or electric vehicle includes a plurality of secondary batteries, and the plurality of secondary batteries are connected to each other in series or in parallel to improve capacity and output.
Meanwhile, the battery pack includes various battery pack protection devices such as a Battery Management System (BMS) in addition to the secondary batteries. These protection devices may play various roles of managing charge and discharge of the battery pack, ensuring safety, or the like. These protection devices may function in consideration of several factors, among which a representative factor may be a voltage of each secondary battery. For example, a specific protection device may prevent overcharge or overdischarge of a secondary battery by using voltage values at both ends of the corresponding secondary battery, and may also perform a balancing function to reduce a deviation of the state of charge among secondary batteries.
As described above, when a protection device included in the battery pack performs a specific function, it may be very important and essential to sense a voltage of each secondary battery included in the battery pack. For this reason, an existing battery pack mostly employs a configuration for detecting a voltage of a secondary battery.
However, the configuration for detecting a voltage of each secondary battery, employed in an existing battery pack, has a very complicated structure and is not easily assembled. In addition, due to such structural complexity and uneasy assembling, it is very difficult to sense a voltage of a unidirectional cell whose positive electrode lead and negative electrode lead are exposed at the same side. Moreover, the existing sensing structure is vulnerable to vibrations, and thus if the battery pack is applied to a device with a lot of vibrations, for example a vehicle, troubles may occur frequently.